1. Field of the Invention
The present invention relates to a scroll pump that includes plate scrolls having nested scroll blades, and tip seals that respectively provide a seal between the tip of the scroll blade of one of the plate scrolls and the plate of the other plate scroll. The present invention also relates to a method of maintaining a scroll vacuum pump including assessing the pump to determine whether a tip seal of the pump should be replaced.
2. Description of the Related Art
A scroll pump is a type of pump that includes a stationary plate scroll having a spiral stationary scroll blade, and an orbiting plate scroll having a spiral orbiting scroll blade. The stationary and orbiting scroll blades are nested with a clearance and predetermined relative angular positioning.
The orbiting scroll plate and hence, the orbiting scroll blade, is coupled to and driven by an eccentric driving mechanism so as to orbit about a longitudinal axis of the pump passing through the axial center of the stationary scroll blade. As a result of this orbiting motion, a series of pockets is delimited by and between the scroll blades. The orbiting motion of the orbiting scroll blade also causes the pockets to in effect move within the pump head assembly such that each of the pockets is selectively and sequentially placed in open communication with an inlet and outlet of the scroll pump, and the volumes of the pockets to vary as the pockets are moved.
More specifically, in an example of such a scroll pump, the motion of the orbiting scroll blade relative to the stationary scroll blade causes the volume of a pocket sealed off from the outlet of the pump and in open communication with the inlet of the pump to expand. Accordingly, fluid is drawn into the pocket through the inlet. Then the pocket is moved to a position at which it is sealed off from the inlet of the pump and is in open communication with the outlet of the pump, and during this time the volume of the pocket is reduced. Thus, the fluid in the pocket is compressed and thereby discharged through the outlet of the pump. In the case of a scroll vacuum pump, the inlet of the pump is connected to a system, e.g., a chamber, from which fluid is to be evacuated with the aid of the scroll pump.
Furthermore, each of the spiral scroll blades of a scroll pump has a number of turns or “wraps”. The exact form of the spiral and the number of wraps dictate the number of pockets formed in series at any given time during the above-described compression process.
In this respect, the sidewall surfaces of the stationary orbiting scroll blades do not contact each other to maintain the pockets. Rather, minute clearances are maintained between the sidewall surfaces at the ends of each pocket after the pocket has been moved out of open communication with the inlet of the pump. Also, the tips of the spiral scroll blades and the opposing plates are spaced apart by minute axial clearances at the top and bottom of each pocket.
Oil may be used to create a seal between the stationary and orbiting plate scroll blades, i.e., to form seals that delimit the pockets with the scroll blades. On the other hand, certain types of scroll pumps, referred to as “dry” scroll pumps, avoid the use of oil because oil may contaminate the fluid being worked by the pump. Instead of oil, dry scroll pumps rely on the small radial clearances maintained between the sidewall surfaces of the nested scroll blades, and tip seals for sealing the top and bottom of each pocket.
With respect to tip seals, each tip seal is seated in a groove extending in and along the length of the tip (axial end) of a respective one of the scroll blades (the groove thus also having the form of a spiral) so as to be interposed between the tip of the scroll blade of a respective one of the plate scrolls and the plate of the other of the plate scrolls. Such tip seals wear out over time and thus, require periodic replacement.
Current practice to assess whether a tip seal needs to be replaced generally requires the user to shut down the process being carried out in the system to which the scroll vacuum pump is connected, disconnect the pump from the system, mount a vacuum pressure gauge to the pump, and run the pump until its ultimate pressure is established which may take at least one hour. In many cases, components of the pump have adsorbed process gas from the system and the pump is otherwise loaded with the process gas. In this case, the pump must be “degassed” and this degassing process adds significantly to the time it takes for ultimate pressure to be established. In any case, a value of the pressure of the fluid is read once ultimate pressure is established, and this value is compared with a reference value representative of the ideal ultimate pressure of the pump to determine the amount of internal leakage across the tip seals and hence, whether a tip seal might need replacing.